Field of the Invention
This invention relates to the maintenance of wind turbines mounted on towers located in bodies of water, and in particular both to capsules for transporting workmen or maintenance personnel, tools and parts to and from such wind turbines, to maintenance vessels for transporting such capsules to and from the towers and to structures on the wind turbine towers for facilitating the transporting of the capsules.
Description of the Prior Art
Wind turbines are currently being used or expected to be used offshore in wind farms. Wind farms are essentially multiple wind turbines disposed in the same locale for generating large amounts of electric power. There are presently thousands of wind farms in a number of countries throughout the world producing about 200 gigawatts of electrical energy, and this number is expected to increase tremendously over the coming decade as low cost oil supplies are depleted and fear of nuclear power increases due to accidents. In China alone where air pollution due to coal-fired power plants is affecting vast areas, they expect over the years to produce near 750 gigawatts of electrical energy from wind farms. Many wind farms are offshore since there are less obstructions to the wind than on land, so that the average wind speed is considerably higher over open water. However, offshore wind farms are more expensive to build than are wind farms on land, and the maintenance costs are relatively higher, particularly in salt water, since the salt water and sea spray are corrosive towards most of the components of wind turbines. Most wind farms that are offshore have wind turbines described as fixed-bottom turbines, that is, having their support towers founded on the sea floor. More recently, floating wind turbines anchored to the sea floor in even deeper water have been constructed. Power is transmitted from offshore turbines by means of undersea cables.
Amongst the conditions which have to be accounted for in an offshore wind farm are waves. Waves are generally under 2.5 m, and the average should be considered to be 3 m. However, high wave conditions occur occasionally, and the maximum survival waves have been determined to be 9.7 m. Currently, for safety reasons, wind turbine maintenance should currently generally not be attempted when the swell conditions exceed 1.5 m.
There are number of maintenance systems currently employed for wind farms. One is the step transfer system. In this system, a vessel sails from a port and conducts operations and maintenance on the respective wind turbines as required. Personnel step off the vessel onto a ladder on the tower holding the wind turbine and attach themselves to a sliding safety harness between the two vertical poles of the ladder and then climb upwards as required. The maximum wave height is usually 1.5 m for safe transfer using the step transfer system. The step transfer system is widely used, and relatively simple and cost effective. However, there are safety implications, and it can be difficult to perform the operations and maintenance required during periods when there are high currents or wave heights above 1.5 m. The occurrence of periods of high waves could delay access and prevent departure increasing the total time to perform the operations and maintenance, and trapping the limited number of maintenance personnel on board the wind turbine tower apparatus, rendering the step transfer system inefficient. It is desired that turbine wind towers should be accessible about 95% of the time, but it has been found that the step transfer system is operable about 75% of the time in the summer and below 45% of the time in the winter. It has been judged that the step transfer system is unlikely to be the best method for use in many wind farms.
Another system involves the use of lifts and cranes, and this is presently the industry standard for transferring tools and equipment to the workmen on offshore turbines. Lifts and cranes are used to transfer equipment, but rarely personnel, all in relatively good weather.
Another system is called the “Waterbridge,” which is an inflatable bridge attached to a vessel and presented to a wind turbine through the attachment of a cable to the foundation of the wind turbine. The cable is kept taut with a constant tension winch. The object of the technology is to absorb the vessel motions through passive use of the inflatable bridge, and the vessel would have the same motion as it would if it were at anchor. In use, a vessel approaches the foundation of the wind turbine, and a cable is hooked over a set of upward curved “Rhino Horns” on a transition piece. The cable is tensioned and the vessel retreats from the wind turbine. An inflatable bridge is then winched up the cable to make positive contact with a ladder on the turbine tower. Personnel are then transferred across the bridge to the ladder. The Waterbridge is cost effective in that it can be deployed when necessary from a vessel and only one platform is needed for all turbines. It can be retrofitted to an existing access boat. A fall arrest lanyard is required for safety purposes. However, only limited field tests have been performed with the Waterbridge. Another shortcoming is that transfer in marginal wave swell conditions may be difficult because the Waterbridge is fixed at the turbine, and it will still be moving up and down significantly at the boat end, because it only uses passive damping of the waves.
The Ampelmann is a stand-alone offshore access system. The Ampelmann must be used with a large vessel. It is quite sophisticated using a reverse flight simulator to compensate for the motion of the waves beneath the boat. The Amplemann has been used in the oil and gas sector. It can be used on any vessel which is more than 50 m in length and can be used by a small crew. It does not require dynamic positioning or adjustments to the vessel. However, it is very expensive and may not be cost effective. It is also quite complicated, having complex moving parts requiring high maintenance. Another problem is the possible leakage from the hydraulics.
The Momac Offshore Transfer System 500 (“MOTS 500”) is a self-stabilizing system for providing safe access to offshore structures by actively compensating for motions of the vessel, rather than passive compensation through hydraulics and damping. MOTS 500 uses proven robotics technology and real-time motion measurement equipment. It can be installed on small and fast vessels, and can be used on existing transfer structures without modification. It has been found to be safe, even in the case of power breakdown or other failures, and requires low maintenance and has a seaworthy construction. The problem with MOTS 500 is that is has not been previously tested in an offshore wind project. It cannot be retrofitted to an existing vessel and it must be able to carry a minimum of three tons at the stern/bow.
An Offshore Access System/Offshore Transfer System (“OAS/OTS”) is essentially a combination of the Waterbridge and the Ampelmann. The OAS/OTS is a passive bridge extending from the boat to a turbine, and is anchored against the turbine in the same way as is the Waterbridge, and is deployed in a manner similar to that of the Ampelmann. It is effective in that it is secured to the ladder at the turbine tower, and it allows greater time to secure fall arrest lanyards at the latter. However, the OAS/OTS is bulky, requiring a large vessel. It only offers a small improvement over existing practices for significant wave heights. The transition piece would probably require several modifications. It has not been used in offshore wind turbines, and could be very costly.
The Small Waterplane Area Twin Hull (“SWATH”) is another possible means for maintaining wind turbines. This system uses a special type of catamaran which is a very stable vessel. The previously described step transfer system can take place with a catamaran or a specially designed platform for installation on a catamaran vessel when personnel are to be transferred to the turbine. SWATH has been used for a number of years in the North Sea, and could be used with a deployable Reinforced Inflatable Boat (“RIB”). Furthermore, the catamaran would reduce the incidence of seasickness. However, a catamaran does have a large draft of about 3 m which would limit its shallow water operation. Furthermore, the proposed catamaran would be 29 m long, which is quite long and would increase the operating expense of the system.
Another possible system for maintenance of offshore wind turbines is Safe Offshore Access (“SASH”, a Swedish acronym). SASH is docking system wherein personnel boarding a fixed structure can step onto it by taking only one step from one horizontal plane to another. This procedure minimizes the time when the boarding personnel are exposed or vulnerable to risk. The boat is an integrated part of the system and facilitates the transfer to the fixed structure because of its raised platform. The system itself has several raised platforms, for stepping onto a raised platform of the wind turbine tower. The boat can move 180° around the docking point between the boat and the tower. The complete SASH system has two diagonally mounted piles per wind turbine so that the boat can always meet the sea bow on. The bow and fender of the boat make it possible to use the boat's engine to control the friction needed to keep the boat stable in terms of the rolling and vertical movements. The boarding personnel do not need to jump or climb, but are able to walk from one fixed structure to another without stress and without any time pressure. This is a fast and improving method. However, since there is only a single hinge point between the pressure point, this magnifies the gap between platform and vessel during high sea states which may render this system unsafe. Also, the SASH system may not be applicable to all foundation types. It is not adapted for tides, which would seem to be necessary. It is currently only used on a Swedish lighthouse and requires a high level of skill of the vessel's skipper.
A Sliding Ladder (“SLILAD”) is a turbine mounted passive system from Momac GmbH & Co. KG, a German company that produced the MOTS 500 discussed earlier. In operation, the SLILAD is fixed to the vessel so that there is no relative movement when the personnel step across the ladder. Once the personnel are securely on the SLILAD, it becomes fixed to the platform and the personnel are able to climb up it if there is no relative movement between the SLILAD and the platform. Since SLILAD has automatic tide level adjustment, there will be no growth of mussels or vegetation on the used part of the latter. The SLILAD has a simple and seaworthy construction, and it is easy to use. However, Momac is no longer developing the SLILAD so it may not be hereafter commercially available. There is an expense involved in maintaining SLILAD and there is a risk of damage due to the large number of moving parts.
Helicopter transfer is well-known. A heli-hoist pad is installed on each wind turbine. Personnel and equipment are winched down one at a time. A maximum of five technicians can be transferred using a helicopter. Helicopters are expensive, and although they can be operated with many kinds of sea-state, they certainly would be dangerous in inclement weather or if the wind turbine is operating. Helicopter transfer is fast, but expensive and the number of personnel and amount of equipment that can be carried per trip is limited. There are risks, health and safety concerns. Helicopters have higher maintenance requirements, are relatively energy inefficient, and are limited in operative range.
Another possible system is the Personnel Transfer System (“PTS”) which is a crane and winch system which is only being developed at this time. It is operated remotely and involves a vessel with fuzzy logic control. It can transfer one load of equipment and one person to the turbine. There have not been any instances of this technology being utilized, although there is at least in one study in which it has been considered. Among its strengths are that there is no risk associated with climbing the transition piece ladder as the PTS lifts. The PTS could be retrofitted to existing vessels, and there is no mechanical contact between the vessel and the turbine. However, amongst its weaknesses is that only one person could be transferred at a time, rendering it slow and involving significant waiting times for persons waiting transfer. Also, a person would not feel safe when suspended several meters above the sea being only supported by a harness, so that survival suits would be necessary. More importantly, this type of maintenance system is only in its preliminary stage, and it is not ready for operation.
A recent development is the Houlder's Turbine Access System (“HTAS”). It is essentially a passive damping mechanism similar to the OAS/OTS discussed earlier, but on a smaller scale which could be fitted to small vessels. It has a unique tuned damping system to reduce the vessels motion response at the bow, but does not attempt to maintain the bow stationary relative to the tower. An access ramp is heave and roll compensated to provide a constant transfer position relative to the tower, either by way of a ladder or platform depending on the tower configuration. The HTAS has been shown to provide for safe transfer at wave heights of 2 m without any relative movement between the access ramp and platform position. Amongst its advantages are that it would require small adjustment to existing procedures and vessel designs, that it is relatively inexpensive and may be economically more favourable than the previously described SWATH and other systems. However, the only increase is the safe access for swell heights exceeding by 0.5 m from 1.5 m to 2 m, but it comes from a company without sufficient reputation or experience, and would require a lot of testing.
A new proposal for maintaining offshore wind turbines is a wave deflection harbour. The purpose of this device is to eliminate waves entirely. It would fit over a transition piece and be attached to a bearing which rotates freely around the position piece according to the direction of the current. When a boat approaches, the deflection harbour would be able to lock into position by use of a remote control operated braking mechanism, similar to that used to stop turbine blades on some wind farms. The wave deflection harbour is a pair of walls which are flat and meet at a point, with the separated walls being connected by a curved wall. The proposed design moves the stagnation point back further into incoming water so that the water will attempt to reattach further from the transition piece, and when it does, it would actually help the vessel into the local harbour. The free rotation of the wave deflection harbour assures that it will be in the correct position for desired flow conditions to occur. The biggest design constraint is the force that is exerted on the foundations. Amongst its advantages are that it could save costs in the long run, that it could increase the size of allowable sea swells and could be applied in many wind farms. However, while the wave deflection harbour is still at its design stage, it may not be cost effective, it would add to capital expenditures, it would require more time through research, testing and prototyping, it may not be operable with some foundation types and the waves may come from a different direction than the current, and could have an adverse effect on the local sea state conditions.
An offshore wind farm maintenance vessel has been prepared by Offshore Ship Designers, an Anglo-Dutch company. It is intended to improve options of deep water wind turbines, reduce maintenance costs and carbon emissions. A mother ship remains in offshore deep water wind farms and has a number of catamaran workboats which carry wind turbine engineers to service the wind turbines. It is a submersible dock ship intended to accompany the foregoing engineers, as well as a crew, service personnel, ships and a support crew. It is further intended to remain offshore rather than reporting to port, and workboats are deployed from the dock ship. It is intended that the fast catamaran and monohull workboats go out from shore to wind farms closer to shore, but not for deep water wind farms. The dock ship is also intended to support Autonomous Rescue and Recovery Craft which are safe watercraft and can support marine and helicopter operations remote from the mother ship in emergency or rescue operations, limited only by their rough weather capabilities. The largest mother ship is intended to accommodate up to 200 engineers and would have extensive recreational, catering facilities and a waste handling plant. A support vessel is intended to carry twenty five wind turbine engineers and carry fuel, potable water, dry and refrigerated storage containers. It is supposed to have a crane, a walkway and two daughter workboats. This concept appears to be very extensive.